The present invention relates to apparatus for contouring the edge of a thin, disc-like workpiece. More particularly, the present invention relates to apparatus for beveling or rounding any sharp corners adjacent the edge of such a workpiece.
In the field of semi-conductor device fabrication, two of the basic components are silicon wafers and photo-masks. The silicon wafer forms the base upon which numerous other components are deposited, etched or formed by other well known techniques. This wafer, typically either two, three or four inches in diameter, is obtained by slicing an elongated bar of pure silicon, properly prepared, in a direction normal to the bar axis. Such wafers, generally between 0.015 and 0.030 inch thick, are very brittle and are susceptible to breakage during the numerous processing steps required to produce semi-conductor devices. The wafers are particularly susceptible to fracturing under the influence of stress raisers such as sharp corners and chips in the edge of the wafer. Breakage resulting from the presence of such stress raisers may result in the scrapping of a fully prepared wafer having hundreds of microcircuits formed thereupon, thus materially reducing the yield and increasing the cost of the component fabrication process.
Numerous attempts have been made to alleviate this problem of wafer breakage. These attempted solutions have addressed both the wafer handling techniques and also some attempts at rounding or beveling the edges of the wafer. The known apparatus heretofor used in such edge beveling attempts has generally comprised a fixed, chisel-like tool having a contoured end which is brought into contact with the edge of a rotating wafer, in a manner similar to a lathe-type turning process. Such apparatus has generally been unsatisfactory for several reasons, primarily relating to irregularities in the edge of the wafer or eccentricities in its rotation. The wafer edge irregularities result both from the method of growing the silicon bar from which the wafer is cut and also from the provision of a flat indexing area customarily cut into a portion of the otherwise generally circular periphery of the wafer. The presence of these irregularities has caused the contoured chisel-type devices both to miss the areas of the flat and to chatter when encountering such irregularities, thus possibly causing breakage of the wafer by the very contouring apparatus itself. Other disadvantages of the prior art apparatus include the necessity for a person operating the apparatus to align the wafer and the cutting tool precisely before each edge contouring operation, a time consuming process whose expense may compromise any economic benefits obtained by reduced wafer breakage.